The invention relates generally to the field of photovoltaic (PV) solar power systems, and more specifically to solar power modules, circuits, and methods for making solar power systems safer for firefighters and installer personnel by reducing the risk of electrical shock.
FIG. 1 is a high level diagram of a conventional PV solar power system 10 to illustrate the need for the invention. The system 10 comprises: two PV strings 11 and 12; an inverter 13; and a cutoff switch 14. Each PV string comprises: a plurality of conventional solar power modules 15 connected in series to produce high dc voltage, typically around 600 Vdc; and a blocking diode, 16 and 17. The inverter 13 converts the dc voltage produced by the two strings 11 and 12 into ac voltage that is output onto the electrical grid 18.
One of the problems with conventional PV systems, such as 10, is the danger of electrical shock. Opening the cutoff switch 14 interrupts the current flowing into the inverter 13, but this does not reduce the risk of shock because the PV modules 15 still produce voltage as long as light falls on them. Rooftop solar arrays are a particular concern for firefighters, who may have to walk on the PV modules 15, or even cut through them with a chainsaw. Even a firefighter standing on the ground may be at risk of electrical shock if he is directing a water hose onto a PV array with exposed high-voltage conductors, because the water stream conducts electricity.
The well-known solution to this problem is module-level shut-down (hereinafter referred to as safe mode), wherein each solar module reduces it's output voltage. In normal operation (meaning, not safe mode) and full sunlight, a solar module typically produces about 30 Vdc. But in safe mode the output voltage typically drops to about 200 mV. A string of twenty modules, each being in safe mode, would produce a total of less than 4 Vdc, which is nonhazardous.
There are products on the market that provide safe mode. These products generally fall into two categories: microinverters; and dc power optimizers. But these devices do more than just module-level shut-down; their main function is Distributed Maximum Power Point Tracking (DMPPT) which can recover some of the energy lost due to mismatches between solar modules. The predominant cause of such mismatches is partial shading, which decreases the energy output of the shaded solar modules.
But only a relatively small percentage of solar installations have enough problems with shading to justify the considerable expense of DMPPT products. What the PV solar power industry really needs is a very low cost, highly reliable means of implementing safe mode, without DMPPT, for all the other solar installations that don't have shading problems.
One significant technical challenge for implementing safe mode is reliable communications, so that every solar module in the array receives the signal to enter safe mode, and this challenge is addressed by the present invention.
Every solar power module has a junction box (j-box) affixed to it's back side. A conventional j-box typically just contains three bypass diodes. In the context of this application, a “smart” j-box is one that contains other circuitry in addition to the bypass diodes. For example, smart j-boxes may include: active bypass, DMPPT, safe mode, performance monitoring, diagnostics, arc flash mitigation, and arc fault detection. Almost all of these functions require some means for the smart j-boxes to communicate in a network.
Presently, smart j-boxes typically communicate either by radio (e.g., ZigBee) or Power-Line Communication (PLC). But unfortunately, neither of these technologies is well suited to PV solar arrays, and consequently they are not always reliable enough for critical safety functions such as initiating safe mode.
Radio networks, such as ZigBee, can experience difficulties (e.g., multi-path fading, and excessive data collisions) because solar modules are conductive and therefore reflect radio waves. The ZigBee protocol (based on the IEEE 802.15 standard) was designed to adapt to reflectors in the environment by forming a plurality of ad hoc peer-to-peer links that find their way around the reflectors. This works well be in many environments, such as offices, homes, or even small solar arrays. But sometimes it doesn't work so well in larger solar arrays because there are just too many transmitters and too many reflectors. Even worse, some solar arrays include sun-tracking mechanisms that change the tilt angle, and hence the reflection angle, of the solar modules as the sun moves across the sky during the course of the day, thereby making reflection problems more likely.
PLC also has problems. For example, some solar arrays such as 10 include multiple strings, wired in parallel for increased current output. When an obstruction 19 (e.g., a tree branch, chimney, or power line) shades one of the solar modules 15 is the first string 11, the voltage produced by the first string 11 is less than the voltage produced by the unshaded string 12, so the first blocking diode 16 is reverse-biased. PLC can't communicate through a reverse-biased diode, so all the modules 15 in the first string 11 lose communication for as long as the shade persists.
The invention overcomes the problem of reliable communications while also drastically reducing the cost of implementing safe mode in solar power modules.